The present application claims priority under 35 U.S.C. xc2xa7 119 to Japanese Application No. 076584/2000 filed on Mar. 17, 2000, which is hereby incorporated by reference in its entirety for all purposes.
1. Field of the Invention
The present invention relates to a surface acoustic wave filter device utilizing surface acoustic waves.
2. Description of the Related Art
With the recent rapid spread of automobile telephones and portable telephones, the necessity for small-sized and high-performance high-frequency Surface Acoustic Wave (SAW) filters has increased.
FIG. 18 shows a conventional SAW filter device 1. The SAW filter device 1 has a SAW filter 2 and a multi-layer ceramic package. The SAW filter 2 has a series arm 3, a first parallel arm 4 and a second parallel arm 5. The first series arm 3 is disposed between a filter input terminal TFi and a filter output terminal TFo, and the first parallel arm 4 and the second parallel arm 5 are disposed between the series arm 3 and a reference potential.
The series arm 3 has a first series resonator S1, a second series resonator S2, and a third series resonator S3. These resonators are disposed in series between the filter input terminal TFi and the filter output terminal TFo. One terminal of the first series resonator S1 is connected to the filter input terminal TFi, and another terminal of the first series resonator S2 is connected to a node n1. One terminal of the second series resonator S2 is connected to the node n1, and another terminal of the second series resonator S2 is connected to a node n2. One terminal of the third series resonator S3 is connected to the node n2, and another terminal of the third series resonator S3 is connected to the filter output terminal TFo.
The first parallel arm 4 has a first parallel resonator P1, and the second parallel arm 5 has a second parallel resonator P2. One terminal of the first parallel resonator P1 is connected to the node n1, and another terminal of the first parallel resonator P1 is connected to a parallel arm reference potential terminal TPg. One terminal of the second parallel resonator P2 is connected to the node n2, and another terminal of the second parallel resonator P2 is connected to the parallel arm reference potential terminal TPg.
The filter input terminal TFi is connected to a package input electrode TPCi via a bonding wire Wi. The filter output terminal TFo is connected to a package output electrode TPCo via a bonding wire Wo. The parallel arm reference potential terminal TPg is connected to a package reference potential electrode TPCg1 via a bonding wire Wg1. The SAW filter device 1 is applied to a band pass filter, therefore it is preferable to decrease signals within an attenuating wave band properly.
However, an input/output impedance of the SAW filter varies because of manufacturing error. An impedance of the package also varies because of manufacturing error. The bonding wires and interconnects on a package have parasitic inductances and parasitic capacitors, which also vary an input/output impedance of the SAW filter device 1. Therefore, a sufficient impedance matching between the package and the SAW filter 2 is not provided in order to achieve a proper characteristic as a SAW filter device.
The present invention is therefore directed to a surface acoustic wave device which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
These and other objects are provided by a surface acoustic wave device including a piezoelectric substrate, a first plurality of resonators which are connected in series and formed on said piezoelectric substrate, a second plurality of resonators which are connected in parallel, and formed on said piezoelectric substrate, an input signal line receiving input signals, a floating electrode being in a floating state, and a wire connecting a point of said input signal line to said floating electrode.